How the OPCW made its conclusion:

In a statement, the watchdog said:

Investigators had "determined that chlorine was released from cylinders" in the Ali Talil neighborhood of Saraqeb in the northern province of Idlib. The organization found two cylinders and an analysis found both had previously contained chlorine.

The organization also found an "unusual presence of chlorine in the local environment."

Investigators also talked to local witnesses and patients at hospitals who had shown symptoms of exposure to chlorine.

Read more: Proof of poison gas: Circumstantial evidence is key

Condemnation

OPCW Director-General Ahmet Uzumcu said: “I strongly condemn the use of toxic chemicals as weapons by anyone, for any reason, and in any circumstances. Such acts contradict the unequivocal prohibition against chemical weapons enshrined in the Chemical Weapons Convention.”

Attack:

The Syrian Observatory for Human Rights, a UK-based war monitor, reported government airstrikes on rebel-held Saraqeb on February 4, adding 11 people had to be treated for breathing difficulties after the attack. Aid organizations later said they had treated people for symptoms of chlorine gas poisoning.

Douma probe ongoing:

Another OPCW investigative team is currently assessing evidence from the Syrian town of Douma to determine whether an April 7 attack that left 40 dead used chlorine and sarin gas. Read more: France: 'All indications' Syrian regime is using chorine gas